Updates for sending files:

* First subpacket of every frame now uses ZCRCW,  this adheres to the following
   "should" from ZMODEM.DOC:
  The next transmitted data frame [after a ZRPOS] should be a ZCRCW frame
  followed by a wait to guarantee complete flushing of the network's memory.

* Added support for the following ZMODEM.DOC requirement:
  FULL STREAMING WITH SAMPLING
  If one of these characters [CAN or ZPAD] is seen, an empty ZCRCE data
  subpacket is sent.
  This is now true, but for ZPAD chars (e.g. ZRPOS header) only.

* Implement an attempt counter in the ZFILE header re-transmit scheme
  (previously, would retry indefinitely until locally or remotely cancelled)

* Better support for receiving a ZSKIP header mid-file transfer.

* Better support for receiving a ZRPOS to end-of-file during a file transfer
  (ala HyperTerminal).

* New and updated log output.